G-7 Flashcards
- Meant by the term segregation
Breaking up of cohesion (separation of concrete aggregate) when using a pump segregation doesn’t occur. Dropping concrete through dense reinforcing steel is the main cause of segregation. Using an extension hose or elephant trunk fed through the rebar allows concrete place without segregation.
- Adequate Vibration to concrete (can under and over vibrate, related to segregation)
Vibrate each layer of concrete fully, particularly around inserts, blackouts and window and door bucks. Vibrate each layer if done in multiple layers, about an inch into the previous layer.
Signs or adequate vibration: as concrete is being vibrated, it will settle as the entrapped air comes to the surface. When the air bubbles stop rising, the surface will have a creamy sheen or appear to glisten.
- External vibrators/forms
External vibration of walls with a hammer or other pounding devices is not adequate to consolidate a well- design low slump concrete.
External vibrators, powered by compressed air, are used with steel forms for making pre-cast concrete members. These external vibrators are attached to the steel form work and are spaced approximately 1200mm apart
- Rounding corners on sidewalk
An edger is used to round the outside corners of the surface of the slab. The edges also creates a decorative border to broom finished sidewalks.
- Vibrator should inserted into the concrete
Vibrator should be inserted to prevent honey comb and bug holes. The top of concrete columns and walls need to be vibrated just as much as the rest of the member to eliminate bug holes
- Ideal temperature range for finishing concrete ( have to cool or heat up more, if it’s too hot it flash steps?!? Makes it weakened, same thing with frozen concrete)
Ideal temperature range for finishing concrete is 15-20 degrees
To cool aggregate spray water
To cool the mix water use refrigeration ?!?
- Using fuel burning heaters on concrete
By using a heated enclosure, concrete work can continue in even the coldest weather. Fuel burning heaters must have their exhaust vented to the outside.
Excessive exposure to carbon monoxide will cause the surface of the concrete to become weak and dusty
- Best/First time to use an edger
Edging is done just after the concrete is floated to move large aggregates away from the forms,
used again before each troweling of the concrete and as the last step or a broom-finished slab
- Purpose of a hand/bull floating
Floating is pushing any large aggregate below the surface of the concrete slab. Edger should be used just after the float has been completed if need for rounded edges.
Purpose: to depress large aggregates
- Bull float first time used
Bull floats are used for large areas.
Long handle makes it so it can be used from a distance without stepping into the fresh concrete.
- Concrete placing again rebar
Using an extension hose or elephant trunk fed through the rebar allows placement of the concrete to be done without segregation.
- Slab - variety of textures and patterns in concrete
Stamped concrete has a surface treatment and is used for sidewalks,patios,and driveways and interior finished floors. Uses stamps that are partially set into the surface of concrete leaving a wide range of patterns and textures
- Extra water added to concrete mix
Added water to the concrete that has begun to set will increase the water and cement ratio thus weakening the hardened concrete.
Only add water to cool the mix
- Added to concrete mix in hot water
Adding hot water to the concrete mixture is often done to warm up the mixtures. Hot water should be added to the aggregate. Cement is only added after checking that the temperature of the water and aggregate mixture is below 38 Celsius
- Wall - preceding layer of concrete - vibrator
Decide how far into the forms the vibrator will have to be placed and mark the depth of the shaft on the vibrator.
Vibrate a minimum of 150 mm into the previous layer of concrete.
- Start placing concrete in a slope
If placing concrete on a slope, place the concrete at the bottom of the slope first and work up the slope, always placing concrete against previous placed concrete.
- When start / finish trowel
Start trowel when floating is complete, finished trowel when surface is smooth.
- Concrete is placed in lifts - wall column
After the last of lift of concrete has been vibrated into place, the surface of the top of the wall is finished. Level strip sets the elevation of the top of the wall, hand float is used to smooth the concrete to the strip.
Maximum spacing of vibrator insertion points when vibrating a wall?
1/2 of radius of influence, 600mm if unknown.
Within how many hours should concrete be placed once water has been added to the mix?
2 hours
If two coats of curing compound are applied, how should the second coat be put on?
at right angles to the first
In order to attain 70% of designed strength, what is the minimum that most concrete should be cured for?
3 days at 10˚C
When patching concrete, why should you key the area to be patched?
to lock the patch to the old concrete
High slump concrete requires:
little or no vibration
When should the curing time for concrete be extended?
if the humidity is low
Which of the following is the easiest to cool in order to reduce the temperature of the concrete mix?
water
What two conditions are required for proper curing of concrete?
moisture and a favorable temperature
What two conditions are required for proper curing of concrete?
moisture and a favorable temperature
